Resistance-size artery and arteriole diameter determines cerebral perfusion pressure and blood flow. A major regulator of cerebral artery diameter is the myocyte intracellular calcium (Ca2+) concentration ([Ca2+]i). An elevation in global [Ca2+]i stimulates vasoconstriction, whereas a reduction in global [Ca2+]i leads to vasodilation. Local (sparks) and propagating (waves) Ca2+ signals also occur in myocytes and can directly and indirectly modulate global [Ca2+]i, leading to changes in arterial diameter. Although research over the past decade has revealed some regulatory mechanisms and physiological functions of local and global Ca2+ signals in arterial myocytes, much remains unclear. Vasoconstrictors that bind to phospholipase C-coupled receptors elevate inositol 1,4,5-trisphophate (IP3) which activates sarcoplasmic reticulum (SR) located IP3 receptors (IP3R), leading to Ca2+ release and an [Ca2+]i elevation. Preliminary data from our laboratory suggest that in addition to mobilizing SR Ca2+, IP3 activates a plasma membrane non-selective cation current (ICat) independently of SR release, and this leads to vasoconstriction. This proposal will focus on investigating the mechanisms by which IP3 regulates cerebral artery myocyte intracellular Ca2+ signaling and arterial contractility. We will investigate 3 specific aims. Aim 1 will test the hypothesis that IP3 activates ICat in myocytes, leading to membrane depolarization, voltage-dependent Ca2+ channel activation, a global [Ca2+]i elevation, and vasoconstriction. Aim 2 will investigate the hypothesis that transient receptor potential (TRP) channels contribute to the IP3-induced ICat and vasoconstriction. Aim 3 will evaluate the contribution of IP3Rs to IP3-induced ICat activation, local and global intracellular Ca2+ signaling and constriction. To investigate these aims, we will use a wide variety of techniques, including laser-scanning confocal and conventional Ca2+ imaging, patch clamp and intracellular electrophysiology, pressurized artery diameter measurements, RNA interference, and IP3R deficient mice. This proposal will provide a better understanding of the mechanisms by which IP3, a principal signaling messenger in the vasculature, regulates arterial Ca2+ signaling and diameter. The resulting data will improve our knowledge of physiological processes that regulate cerebral blood flow, the alteration of which can lead to cerebrovascular pathologies, including hypertension and stroke. PUBLIC HEALTH RELEVANCE It is generally considered that in arterial smooth muscle cells, inositol trisphosphate (IP3) activates IP3 receptors, leading the release of stored calcium (Ca2+) and an elevation in intracellular Ca2+ concentration ([Ca2+]i) that stimulates constriction. Our proposal suggests the novel mechanism that in smooth muscle cells, vasoconstrictor-induced IP3 also activates IP3 receptors, and through a mechanism that does not require sarcoplasmic reticulum Ca2+ release, this leads to the stimulation of a plasma membrane cation current to which TRPC3 channels contribute. The ensuing membrane depolarization activates voltage-dependent calcium channels, leading to an [Ca2+]i elevation, and constriction.